In resent years, there have been proposed projectors which use a laser light source, and since they have a large color reproduction range compared to the conventional projector using a discharge lamp light source, they gather attention as a high quality image display device of the next generation. However, when a coherent light source such as a laser is used, diffraction of light generates a speckle, which is a high contrast spotty pattern. This pattern makes the projected image look as if it is floating and impairs the quality of the image. On the other hand, the speckle itself stems from using the laser, and thus it is difficult in principle to fully remove the speckle. Therefore, it is desired to reduce a speckle to be indistinctive instead of removing it.
The speckle stems from the coherency of the laser, and as a measure to reduce it there are proposed a method for lowering the coherency of the laser light beam and a method for normalizing the speckle by projecting temporally different speckle states.
On the other hand, it is known that the laser light beam is polarized in general and the state of speckle depends on the polarization state (see Patent Document 1). By using this knowledge, a speckle can be reduced by changing the polarization state of the laser light beam to change the speckle state, and superposing these temporally changing speckle states.
In Patent Document 1, for example, there is a polarization control section provided in the optical path between a laser light source and a scanning mirror, and a wave plate provided on the polarization control section is rotated, as the above-described method for changing the polarization state of the laser light beam, to temporally change the polarization state of the projected image. Thus, there is proposed a method for reducing the speckle by multiplexing the changing polarization states.
In Patent Document 2, there are defined regions in matrix, which regions transmit a laser beam therethrough, and in each of which regions a laser beam with different polarization state is generated; and in addition the polarization state is temporally changed to reduce the speckle.